Stability analysis of slug flow control

The threat of slugging to production facilities has been known since the 1970s. This undesirable flow phenomenon continues to attract the attention of researchers and operators alike. The most common method for slug mitigation is by choking the valve at the exit of the riser which unfortunately could negatively affect production. The focus, therefore, is to satisfy the need for system stability and to maximize production simultaneously. Active feedback control is a promising way to achieve this. However, due to the complexity of multiphase flow systems, it is a challenge to develop a robust slug control system to achieve the desired performance using existing design tools. In this paper, a new general method for multiphase flow system stability analysis was proposed. Active feedback control was observed to optimize slug attenuation compared with manual choking. The use of soft sensors was believed to be desirable for the practical implementation of the proposed control technique

Stabilizing slug flow at large valve opening using active feedback control

The threat of slugging to production facilities has been known since the 70’s. This undesirable flow phenomenon continues to attract the attention of researchers and operators alike. The most common method for slug mitigation is by choking the valve at the exit of the riser which unfortunately could negatively affect production. The focus therefore is to satisfy the need for system stability and to maximize production simultaneously. Active feedback control is a promising way to achieve this. However, due to the complexity of multiphase flow systems, it is a challenge to develop a robust slug control system to achieve the desired performance using existing design tools. In this paper, a new general method for multiphase flow system stability analysis was proposed. Active feedback control was observed to optimize slug attenuation compared with manual choking

Experimental investigation of hydrodynamic slug mitigation potential of an intermittent absorber

The need to handle hydrodynamic slugs in a more efficient way becomes important as oil and gas activities shift deep offshore. This study describes the use of a vessel coupled to the pipeline-riser system upstream of the first stage separator for hydrodynamic slug attenuation. The experiments were carried out in a 2″ pipeline-riser system which comprises of a 40 m long horizontal pipe connected to a 11 m high vertical riser followed by a 3 m horizontal topside section. Air and water were used as experimental fluids. Bifurcation maps and slug attenuation index (SAI) have been used to quantify increase in oil production and the slug attenuation potential of this concept. The device was observed to reduce the pressure fluctuations characterising hydrodynamic slug flow up to 22%. The device also provides additional benefits of stabilising the flow at higher valve opening (choke setting) and lower pressure compared to traditional choking. This in practice translates to increase in oil production. Special case of hydrodynamic slugs which exhibit overchoking induced slugging (OIS) was also observed to be relatively attenuated by the introduction of the absorber

Experimental Investigation of Hydrodynamic Slug Flow in Pipeline-Riser Systems

Activities in oil and gas industry have shifted deep offshore. There is therefore the need to envisage and accurately provide for flow assurance challenges that might be encountered throughout the life of a field. Slug flow is one of the flow assurance concerns confronting the industry. The objective of the study was to gain insight into the behaviour of hydrodynamic slug flow in pipeline-riser system. This understanding is needed for the development of appropriate slug control strategy. Experimental studi es were conducted in a 2” pipeline-riser system and a 2” horizontal two -phase flow facility. Slug envelopes were developed for the pipeline-riser system, the vertical and the horizontal pipes. The results revealed three distinct slug flow behaviours. The first type of slug was formed in the horizontal pipeline and transported through the riser pipe nearly unchanged, the second type of slugs were formed in the horizontal pipe but also experience growth in the riser pipe while the third are slugs formed in the vertical pipe without the influence of the upstream horizontal pipe. There is therefore the need to develop appropriate slug control strategies based on the observed behaviour of the identified region

Attenuating severe slug flow at large valve opening for increased oil production without feedback control signal

In the co-current flow of gas-liquid mixtures through pipeline-riser system, severe slugging is frequently encountered and manifests insignificant fluctuation of flow and pressure. This can pose serious threat to production facilities. The most common method for its mitigation is by choking which unfortunately could negatively affect production. The objective of this study therefore is to develop a technique that can help stabilise the system and maximise production simultaneously. In this paper, a new general method for multiphase flow system stability analysis was proposed based on a new passive attenuation method – the intermittent absorber. A series of experiments were carried out in a 4″ pipeline-riser system which is 55m long with horizontal pipe inclined at 2° connected to 10.5m high catenary riser followed by 3m horizontal topside section. Air and water were used as experimental fluids. Numerical studies were also conducted on a 4″ pipeline-riser system to proof the concept. The results showed that the intermittent absorber possess potential for flow at larger valve opening and lower pressure. For the case studied, up to 9% reduction in the riser base pressure was reported which practically implies increased oil production

Characteristics of horizontal gas-liquid two-phase flow measurement in a medium-sized pipe using gamma densitometry

Two-phase flows are common occurrences in many industrial applications. The understanding of their characteristics in industrial piping systems is vital for the efficient design, optimization, and operation of industrial processes. Most of the previous experimental studies involving the use of gamma densitometers for holdup measurements in air-water mixtures are limited to smaller diameter pipes (generally regarded as those with < 50 mm in nominal diameter). Further, very few literature report experimental data obtained using gamma desitometers. This paper presents an application of a gamma densitometer in the measurement of two-phase flow characteristics in an intermediate diameter pipe (nominal diameter between 50 mm and 100 mm). Scaled air-water experiments were performed in a 17-m long, 0.0764-m internal diameter horizontal pipe. Liquid superficial velocity ranged between 0.1–0.4 m/s while gas superficial velocity ranged from 0.3 to 10.0 m/s. The measured parameters include liquid holdup, pressure gradient, flow pattern, and slug flow features. The flow patterns observed were stratified, stratified-wavy, plug, slug, and annular flows. Plug and slug flow patterns showed good agreement with established flow pattern maps. Furthermore, the slug translational velocity was observed to increase with increasing mixture velocity, as reported by previous authors, hence establishing the reliability of the instrumentation employed. The slug body length was also measured using the gamma densitometer and was found to be within the range 24–36D with a mean length of 30.6D

Offshore Topside Rotating Packed Bed as Process Intensified Alternative for Natural Gas Sweetening and Dehydration

This work is aimed at investigating the benefits of replacing conventional process unit operations with process intensified ones in offshore applications. This ensures that better use is made of raw materials, lower energy consumption and a reduced plant volume was achieved. Specifically, a rotating packed bed technology has been used for gas dehydration and sweetening. To achieve the aim of this study, a process intensification approach is used to redesign mature absorption processes to more compact and efficient one. Process simulation using Aspen Hysys was carried out for Triethylene glycol dehydration and monoethanolamine sweetening. More than 36-fold absorption unit size reduction was achieved thereby effecting large decrease in capital and operating costs compared to the conventional packed columns currently utilized in the offshore oil and gas industry. The process intensified technologies therefore can be deployed for offshore applications where space and size considerations are of utmost importance

Topside Pipeline Design for Slug Attenuation and Increased Oil Production

In oil and gas production system, slugging is frequently encountered when gas-liquid mixtures are transported through a common pipeline-riser system. This phenomenon usually manifests in significant fluctuation of flow and pressure which can impact the production system negatively. Topside choking is usually employed as a mitigation technique but with its attendant reduction in production capacity. The objective of this study therefore is to investigate the optimisation of topside pipeline diameter and choking for effective slug attenuation and optimised oil production. In this paper, a new method for slug flow attenuation has been proposed. The potential of using effective topside pipeline- diameter design for slug flow attenuation was theoretically shown. Numerical studies were also done to show that the concept can indeed be adapted for effective slug attenuation using an industrial software. Experimental studies were conducted in a 4” pipeline -riser system to validate the numerical and theoretical studies. The results showed that the optimised design of topside pipe diameter has potential for slug flow attenuation at larger valve opening which effectively translates to lower pressure and increased oil production. For the case studied, up to 49% reduction in the pressure drop across the topside choke valve was reported which practically implied increased flow capacity. An optimum volume which satisfied size, system stability and production constraints was obtained

Prediction of Riser Base Pressure in a Multiphase Pipeline-Riser System Using Artificial Neural Networks

In the multiphase flow of oil and gas in pipeline-riser systems, reliable pressure measurements and monitoring is of utmost importance for flow assurance. These measurements are usually obtained using remote pressure measuring gauges and other devices. They are employed in the automatic slug flow control technique. However, these devices are quite expensive and often require calibration at intervals to guarantee accuracy and precision. There is therefore, the need for suitable alternatives. In this study, a feed-forward back propagation artificial neural network (ANN) for predicting riser base pressure in offshore pipeline riser systems is presented. A total of 16,870 experimental data sets were used to develop the ANN model. The results revealed near perfect predictions with an average mean square error of 0.00207197 and regression correlation coefficient, R values as high as 0.99919. The models obtained from this work can be pivotal to the development of data driven control of slug in pipeline-riser systems

Mathematical Modelling Of Ethanol Dehydration From Azeotropic Concentration Using Pressure Swing Adsorption (PSA) Process

The demand for pure ethanol has become paramount for its various applications in biofuels and preparation of industrial chemicals, food and pharmaceutical products. Pressure Swing Adsorption (PSA) has proven to be an effective process and a more economical method for separating the azeotropic concentration of ethanol-water (95% ethanol, 5% water) than distillation. The modeling of PSA of ethanol-water azeotropic concentration on zeolite 3A was done. The unsteady state mass balance equation for the water molecule in the packed bed was done as well. The sets of Partial Differential Equations obtained from the model equations were solved with the Crank Nicholson finite difference method using MATLAB software. The simulated data was observed to be considerably in agreement with the experimental data.  The data obtained for dimensionless bed length z= 0.1 explains better the experimental result with Least Sum of Square Residuals (LSSR) of 0.06588. Breakthrough time of 600sec was obtained from simulated data while that of experimental was 680sec. The purity of 99.2705% v/v anhydrous ethanol was obtained